It is necessary to use filament heating systems for lamps, in particular fluorescent lamps. In the case of known filament heating systems, it is conventional for them not to be subjected to closed-loop control. In particular in the case of dimmable fluorescent lamps, there is a reduction in the discharge current in an operation in which the light from the lamps is reduced by means of a dimmer. This results in the heating of the filaments likewise being reduced, and it therefore being necessary to carry out additional heating externally. In the case of the known filament heating systems, it is therefore necessary for dedicated ballasts to have been developed, depending on the number of lamps to be operated, since the additional heating was dependent on the number of lamps.